Carbetocin is a Functional Selective Gq Agonist That Does Not Promote Oxytocin Receptor Recycling After Inducing β‐Arrestin‐Independent Internalisation

Abstract

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.Carbetocin, a long-acting oxytocin analogue, has been reported to elicit interesting and peculiar behavioural effects. The present study investigated the molecular pharmacology of carbetocin, aiming to better understand the molecular basis of its action in the brain. Using bioluminescence resonance energy transfer biosensors, we characterised the effects of carbetocin on the three human oxytocin/vasopressin receptors expressed in the nervous system: the oxytocin receptor (OXTR) and the vasopressin V1a (V1aR) and V1b (V1bR) receptors. Our results indicate that (i) carbetocin activates the OXTR but not the V1aR and V1bR at which it may act as an antagonist; (ii) carbetocin selectively activates only the OXTR/Gq pathway displaying a strong functional selectivity; (iii) carbetocin is a partial agonist at the OXTR/Gq coupling; (iv) carbetocin promotes OXTR internalisation via a previously unreported b-arrestin-independent pathway; and (v) carbetocin does not induce OXTR recycling to the plasma membrane. Altogether, these molecular pharmacology features identify carbetocin as a substantially different analogue compared to the endogenous oxytocin and, consequently, carbetocin is not expected to mimic oxytocin in the brain. Whether these unique features of carbetocin could be exploited therapeutically remains to be established. Oxytocin is a small nonapeptide produced by magnocellular and parvocellular neurones of the supraoptic and paraventricular nuclei of the hypothalamus; from these sites of synthesis, the peptide is delivered to the periphery by axons projecting to the posterior pituitary and to the central nervous system (CNS) by dendrites and axonal collaterals (1,2). In the periphery, oxytocin has several functions, including the contraction of uterine smooth muscles during labour, an effect that has been extensively exploited for decades to promote contractions during the third stage of labour and to control bleeding after childbirth (3,4). Oxytocin also promotes the contraction of the mammary myoepithelium during lactation (5) and the intranasal use of oxytocin to stimulate lactation in breastfeeding women was approved by the Food and Drug Administration in the USA in 1960, even though it was discontinued in 1997 for commercial reasons.In the CNS, oxytocin acts as a neurotransmitter/neuromodulator regulating several aspects of social behaviour, learning and memory processes, and stress and anxiety responses (6,7). On the basis of its capability to promote social interactions in all vertebrates, including humans, oxytocin has been proposed as a clinical treatment for relieving social impairments associated with neurodevelopmental and psychiatric disorders (8,9). In the past decade, a number of clinical trials have been performed in autism and schizophrenia but, unfortunately, no consensus on the real efficacy of o...

Oxytocin treatment in children with Prader–Willi syndrome: A double‐blind, placebo‐controlled, crossover study

“…It is known that exposure to oxytocin leads to desensitization of the oxytocin receptor in order to protect cells from overstimulation (Conti, Sertic, Reversi, & Chini, 2009). This desensitization can occur very rapidly, within seconds or minutes, and is observed in the majority of G protein-coupled receptors, including the oxytocin receptor (Passoni, Leonzino, Gigliucci, Chini, & Busnelli, 2016). However, studies have shown that these receptors are recycled back to the cell surface once oxytocin levels have decreased (Passoni et al, 2016).…”

Oxytocin Reduces Ethanol Self‐Administration in Mice

“…Interestingly, the arginine vasopressin 1b (V1b) receptor antagonist SR149415 was shown to reduce ethanol drinking in genetically high alcohol-preferring rats (Zhou et al, 2011) and dependent Wistar rats (Edwards et al, 2012). However, results from a study demonstrating that the synthetic oxytocin receptor agonist carbetocin, which exhibits low affinity for V1b (and V1a) receptors compared to oxytocin receptors (Passoni et al, 2016), effectively reduced ethanol-induced conditioned reward supports a role for oxytocin receptors (Bahi, 2015). Further investigation is needed to determine the relative role of these related neuropeptide receptors in mediating the ability of oxytocin to reduce ethanol reward and drinking.…”

Oxytocin Receptor Signaling in Vascular Function and Stroke

“…Further detection of OXTR co-labeling with Rab5 and Rab4, small GTPases involved in "short cycle" trafficking back to the membrane, also support this conclusion (Conti et al, 2009). While this may be the typical cycle, this is not always the case, as β-arrestin-independent internalization and recycling loss has also been reported, particular with analogs of OXT (Passoni et al, 2016). Some nuclear trafficking of the OXTR in concert with β-arrestins, Rab5, importin-β, and transportin-1 has been reported in mouse osteoblasts, but any transcription modifying effects remain speculative (Di Benedetto et al, 2014).…”

“…Given the established role the secretory pathway plays in controlling the number of receptors available at the cell surface, and therefore signaling magnitude (Dong et al, 2007), further research into OXTR trafficking dynamics will shed insights into its potential multifactorial role in cell and tissue function. Possible variances in desensitization and trafficking of the OXTR could have therapeutic implications based on drugs and individual differences, such as the divergence in pathways by different agonists (e.g., carbetocin vs. OXT) (Passoni et al, 2016). These variables may impact our understanding for OXTR-mediated functions with respect to vascular health, as discussed below.…”